During critical illness the demand for glutamine is increased. Rapid depletion of glutamine stores in critically ill patients has been described and correlated to increased mortality. Glutamine depletion may be deleterious in critical illness because of adverse effects on the essential functions mentioned above. For example glutamine depletion may cause gut mucosal barrier function to deteriorate, leading to bacterial translocation and enhanced systemic inflammatory response with increased risk for multisystem organ failure. Clinical trials performed in a wide range of patients with serious illness, including cancer, trauma, burn, major surgery and critical illness, have demonstrated possible benefits of glutamine supplementation. Interpretation of the results of multiple studies is made difficult based on differences in glutamine dosing, route of administration, population studied, and endpoints used.